CT for Diagnosing Lung Disease in Foals

"Neonates are small enough that just about any part of the body can be imaged (with CT)," said Lascola.

Photo: Kara M. Lascola, DVM, MS, Dipl. ACVIM

Equine veterinarians are taking a page from human medicine books as they begin examining applications for computed tomography (CT) in evaluating pulmonary disease in foals. Kara M. Lascola, DVM, MS, Dipl. ACVIM, an assistant professor at the University of Illinois at Urbana-Champaign College of Veterinary Medicine, described related research and potential challenges at the 2013 American College of Veterinary Internal Medicine Forum, held June 12-15 in Seattle, Wash.

Computed tomography is an imaging method that creates a cross-sectional and detailed picture of structures inside the body using a series of radiographs (X rays). Lascola explained that CT is the preferred imaging modality for numerous human pulmonary problems, including emphysema, acute respiratory distress syndrome, and airway disease. Currently, veterinarians typically employ radiography to evaluate equine lungs for disease; however, CT images' higher resolution makes it easier to identify pathologic processes, she said.

"Current CT imaging technology allows for whole lung imaging within seconds and provides very high quality resolution, allowing for more accurate anatomic and morphologic characterization of all components of the lung," she explained.

Pulmonary disease is a significant cause of illness and death in young foals, she said, which "highlights the need for species- and age-specific characterization of normal lung CT morphology in addition to disease characterization." And although this modality has been widely described in dogs and cats, there's just one published report—a case study—describing the use of thoracic CT in an older foal, Lascola relayed.

CT: Not Just for Lungs

Evaluating neonatal foals' lungs is just the tip of the iceberg when it comes to using computed tomography (CT) in young horses.

"Neonates are small enough that just about any part of the body can be imaged (with CT)," said Kara M. Lascola, DVM, MS, Dipl. ACVIM, an assistant professor at the University of Illinois at Urbana-Champaign College of Veterinary Medicine. "So, in addition to the lungs, other parts of the respiratory system like the upper airways can be imaged, as well as the rest of the thorax, the limbs to evaluate orthopedic problems, septic joints, osteomyelitis (infection of the bone). We have used CT to diagnose skull fractures and to evaluate other abnormalities of the head."

Lascola also said veterinarians can use CT in mature horses and older foals; however, there are limitations to consider.

"The first (limitation) is the weight of the animal," she explained. "Standard CT tables only accommodate up to a 350-450 pound animal. Many large animal referral centers with CT now have special tables that will accommodate more than 1,000 pounds, which means that older juvenile and adult horses can be imaged.

"The second limitation is the size of the animal," Lascola said. "Although special tables can take the weight, the size of the CT machine does not change and it is not possible to fit the chest through the gantry of the machine. This means that for older juvenile and adult horses we are limited to imaging the head (brain, sinuses, nasal cavity, teeth, and upper airway), upper neck, and limbs."

Despite the limitations, Lascola said "the ability to use CT for imaging of these areas is a great asset."

Erica Larson

A Pilot Study

Lascola and colleagues recently completed a pilot study, funded by the Morris Animal Foundation, using CT images to characterize lungs in healthy, sedated equine neonates breathing spontaneously (on their own, without assistance from a ventilator). In that study, she said, the team found that veterinarians can perform CT imaging rapidly when foals are sedated, and the foals tolerate the procedure well. Morphologic (structural) findings included:

All foals showed atelectasis (failure of a lung to expand properly--essentially, lung collapse), characterized by a "patchy alveolar radiographic pattern" in the dependent part of the lung (Lascola explained that the dependent part of the lung is the lowest part. For instance, she said, if a foal is laying on its right side, the right lung is the lowest and the right lung lobe is the dependent part. "Because these foals were sternal (i.e., weight on base of chest and abdomen), the lowest part of the lung was the dependent part. We referred to it as the ventral lung," she said.); this corresponded with reduced gas (air) volume in the lower lung compared to the normally aerated non-dependent lung.

Foals younger than seven days had a decreased volume of normally or well aerated lung compared to older foals.

The atelectasis in younger foals was considered more severe than in older foals.

"Given these findings, use of CT to evaluate changes in lung density and volumes in individual foals over the first two weeks of life could provide more specific information regarding potential age-related changes," she said.

While CT appears to be a useful tool for imaging equine neonates' lungs, Lascola noted that her study identified two potential limitations to this diagnostic modality in spontaneously breathing foals: the risk of atelectasis and the inability to control at which phase of breathing the CT image is taken.

"Atelectasis can sometimes be observed with standard radiographs, as well," Lascola explained. "However with CT … the use of sedation or general anesthesia is much more common. It is not uncommon for these medications and procedures to predispose humans and animals to atelectasis. There is also some evidence that neonates are more prone to developing atelectasis possibly because of immaturity. The amount of atelectasis is often quite mild especially when the procedure is done quickly. In healthy animals it also resolves quickly as well. "

She also noted that veterinarians have options for combating the latter issue, including controlling the foal's breathing with a manual or mechanical ventilator or using recruitment maneuvers to open collapsed portions of the lungs.

"Recruitment maneuvers are used to 'open-up' the collapsed (atelectatic) airways," Lascola said. "They usually involve giving on big breath or a series of breaths to the patient. Once the maneuver is performed the CT scan is usually taken with the patient being held during maximal inspiration—just when the biggest part of the breath is given and before the patient exhales. This opens the collapsed airways and holds them open during the scan. With the new machines it takes about 20-25 seconds to perform a complete CT scan in a foal and so most of these recruitment maneuvers are very safe to the animal."

Continuing Research

Lascola's team continues to study CT use for evaluating neonatal foals' lungs.

"We are completing a study using recruitment maneuvers and positioning the foals on their back (dorsally) versus on their chest (sternally) to minimize atelectasis," she told The Horse. "The most important things are to find a technique that is fast, safe, and clinically applicable for our patients; provides the best image quality for diagnosis; and provides other people who may use CT with information regarding potential limitations or causes of artifacts that may need to be accounted for when interpreting the images."

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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